1. Field of the Invention
The present invention relates to a testing circuit, and more particularly, to a circuit testing apparatus capable of testing an analog signal accurately.
2. Description of the Prior Art
With developments of technology, the functions of an integrated circuit (IC) become more and more complicated. In addition to analog-signal integrated circuits (ICs) for purely processing analog signals, and digital-signal ICs for purely processing digital signals, multiple ICs capable of processing analog and digital signals are also available, and are usually called mixed-signal ICs. In general, each IC, whether it is a digital-signal IC, an analog-signal IC or a mixed-signal IC, will be tested after its fabrication process to ensure the quality. Manufacturers determine whether an IC can be sold to customers according to the test results. Taking an example of a common IC testing scheme, a mixed-signal tester is usually utilized for testing ICs after the fabrication process. Current mixed-signal testers are very expensive; therefore, if the mixed-signal testers are utilized for testing analog-signal ICs or mixed-signal ICs, a long testing period is required and the total cost increases significantly.
Additionally, in comparison to testing digital-signal ICs, testing analog-signal ICs and mixed-signal ICs is more complicated, and accuracy requirements often need to be higher. The accuracy of a conventional testing scheme for testing analog signals generated from analog-signal ICs (or mixed-signal ICs) is not good enough, however. The accuracy is often down to only 10 mV˜20 mV. If an analog signal under test is amplified through a system by +60 dB˜100 dB, noise existing in the analog signal under test may be amplified from a voltage level below 10 mV to a voltage level above 10V, and the system will be influenced by the amplified noise. In other words, a test result may not be reliable. It is important to provide a testing scheme capable of supporting a higher accuracy requirement to increase reliability of the system.